Relay satellite for Chang'e-4 lunar probe enters orbit

A Long March-4C rocket carrying a relay satellite, named Queqiao (Magpie Bridge), is launched at 5:28 a.m. Beijing Time from southwest China's Xichang Satellite Launch Center, May 21, 2018. [Photo: cnsa.gov.cn]

The relay satellite for the China's planned Chang'e-4 lunar probe, which is expected to land softly on the far side of the Moon at the end of this year, has entered its mission orbit.

After a journey of more than 20 days, the satellite Queqiao, which means Magpie Bridge, entered a halo orbit around the Earth-Moon L2 point at around 11 a.m. Beijing Time on Thursday.

The successful launch of Queqiao has laid a solid foundation for the Chang'e-4, which is expected to become the world's first probe to soft-land and rove on the far side of the Moon.

Since the Chang'e-4 lunar probe will be on the far side of the Moon from Earth, it can't directly communicate with its controllers on the ground. This is why Queqiao was launched, to act as a communications relay.

Wu Weiren, chief designer of China's lunar exploration program, explains:

"It has three major functions. Firstly, tracing and supervising the whole process of the landing and roving of the Chang'e-4 lunar probe on the far side of the moon in the latter half of the year, and sending all its data back. Secondly, monitoring the condition of the Chang'e-4 lunar lander and rover, which will be staying on the moon for long period of time. Thirdly, all of the data from the scientific exploration of the far side of the Moon will be transmitted back to the Earth through the relay satellite. As a result, the relay satellite is very important, it is indispensable and vital for the success of the whole program."

The satellite is orbiting about 400 thousand kilometers from Earth and 65 thousand kilometers from the Moon.

Regular adjustments will be made to Queqiao's orbit to ensure that it can continue to provide clear communication between Earth and the Chang'e-4 lunar probe.

The relay satellite project has been viewed as a mark that China's space technologies are among the most advanced globally in terms of in-orbit control and space communication.

Wu Weiren adds that a series of in-orbit tests and relay communication tests will be conducted with Queqiao in the coming months.

"In the follow-up work, we will conduct accuracy tests to make sure that the satellite can communicate with the Earth at a distance between 460 thousand kilometers and 480 thousand kilometers. Large diameter antenna will be used on Earth during the tests to guarantee a high level of accuracy."

The Queqiao satellite, weighing about 400 kilograms and with a designed life of three years, carries several antennas. 

One, shaped like an umbrella with a diameter of 4.2 meters, is the largest communication antenna ever used in deep space exploration.

Scientific instruments designed by the Netherlands and Saudi Arabia are also aboard the Queqiao.

The lander and rover of Chang'e-4 will be equipped with instruments developed by Germany and Sweden. 

The satellite could stay in its halo orbit for a long time using relatively little fuel, thanks to the relatively small influence of gravity around that part of the moon.